1. Field of the Invention
This invention relates to a method for obtaining information about interstitial patterns of the lungs, which information is to be utilized in making a diagnosis of the lung tissues. This invention also relates to a method for obtaining information about density of trabeculae of a bone of a human body, or the like, which information is to be utilized in making a diagnosis of an osteoporosis.
2. Description of the Prior Art
Techniques for reading out a recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (i.e., an image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a photocopy, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like, can be reproduced.
Also, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
In the radiation image recording and reproducing systems wherein recording media, such as X-ray film or stimulable phosphor sheets, are used, various kinds of image processings are ordinarily carried out on the detected image signal such that a visible image may be reproduced which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness.
Also, with the radiation image recording and reproducing systems wherein recording media, such as X-ray film or stimulable phosphor sheets, are used, a radiation image of the chest of a human body is obtained and utilized in making a diagnosis of a disease of the lungs of the human body. Specifically, image signal components representing the image patterns of the lungs are sampled from the image signal representing the chest image. Also, the position of a region of interest is determined, and the state of a disease of the lungs in the region of interest is investigated.
Of the diseases of the lungs of human bodies, which diseases are thus found, interstitial diseases, i.e. interstitial abnormal states in the ventricles of the lungs due to accumulation of liquids or protein substances, occur most frequently. Examinations of the interstitial diseases of the lungs reach 40% of the X-ray examinations carried out in hospitals in the United States of America. For example, it has been reported that 22% of the abnormal states of the lungs found with the recording of X-ray images of the chests in the Medical Center of Chicago University are due to interstitial abnormal states.
The rating of interstitial diseases utilizing the X-ray photographs of the chests is the most difficult technique in the radiotherapeutics. This is because various patterns and complicated displacements are encountered in the interstitial diseases, because the relationship between radiological findings and pathological findings has not yet been established, and because the terms used in order to express X-ray image patterns have not yet been defined clearly and vary for different experts in the radiology. Therefore, the manner, in which interstitial patterns of the lungs are rated, vary for different persons, organizations, and the like. Also, even if the interstitial patterns of the lungs are rated by a single person, the results of the rating will vary for different time and circumstances. Accordingly, accurate diagnoses of interstitial diseases of the lungs could not be made in the past.
In view of the above circumstances, various methods have been proposed wherein interstitial patterns of the lungs are quantitatively determined such that the interstitial patterns can be rated objectively. For example, a method for rating the interstitial patterns of the lungs in accordance with statistical properties of a density distribution in an X-ray image has been proposed by Sutton in "IEEE Transactions on Computers," Vol. C-21, No. 7, July 1972. Also, a method for obtaining power spectra of the lung tissues by use of engineering Fourier transformation and discriminating between the normal lungs and the lungs affected by interstitial diseases has been proposed by Tully, et al. in "Investigative Radiology," July-August, 1978, Vol. 13, pp. 298-305. Further, a method, wherein tissue patterns are encoded in accordance with the directions of gray level gradient vectors, which are determined by carrying out a sampling operation on an X-ray photograph of the chest at 1.2 mm sampling intervals, such that the severity of a pneumoconiosis may be investigated, has been proposed by Jagoe, et al in "British Journal of Industrial Medicine," 1975, 32, pp. 267-272.
However, the proposed methods have the problems in that, in cases where the contrast of the patterns due to an interstitial disease is low, a small change in density on the X-ray photograph is lost, and therefore an accurate diagnosis of the interstitial disease of the lungs cannot be made. Such that these problems may be eliminated, a novel method has been proposed in Japanese Unexamined Patent Publication No. 1(1989)-125675. The proposed method comprises the steps of setting a region of interest on a chest image, removing image signal components, which represent the background information, from the image signal components corresponding to the region of interest, processing the image signal components, which are now free of the background information, with a spectral analysis, and thereby automatically detecting and assaying the interstitial patterns. With the proposed method, the coarseness or fineness of a texture can be expressed by the level and the root-mean-square value (rms value) of the first-order moment of a power spectrum obtained from the spectral analysis. Therefore, the state of an interstitial disease of the lungs can be expressed quantitatively.
However, with the method proposed in Japanese Unexamined Patent Publication No. 1(1989)-125675, the region having a predetermined range is set on a radiation image, and the mean-level spectrum in the predetermined range is obtained. Therefore, of the information representing the radiation image, only the information corresponding to the predetermined range can be obtained. Accordingly, with the proposed method, local spectral analyses of interstitial patterns of the lungs cannot be carried out. Also, with the proposed method wherein the spectral analysis is carried out, the region is set manually. Therefore, the problems often occur in that the region is set at an incorrect location. In such cases, accurate information about the interstitial patterns of the lungs cannot be obtained, and the severity of a disease of the lung fields cannot be found accurately. Further, with the proposed method wherein the background information must be removed from the image signal, the calculation time cannot be kept short.
Recently, with the rapid increase in the number of aged persons, osteoporosis has become a medical and social important problem. It has been indicated that the early detection and early diagnosis are of great importance in osteoporosis as in various other diseases. In order for the osteoporosis to be detected early, it is necessary to make a system, with which examinations of a large number of persons can be carried out easily and accurately such that the bodily and economical burdens to the persons may be kept as light as possible.
A conventional method for making a diagnosis of the osteoporosis will be described hereinbelow. The conventional method is referred to as the Jidai's classification method. With the conventional method, the severity of the osteoporosis is classified as shown in FIGS. 10A, 10B, 10C, 10D, and 10E in accordance with the impressions which a physician received during observation of, primarily, the trabeculae of a vertebral bone or the density of the image pattern of the vertebral bone. Specifically, FIG. 10A shows an X-ray image of a normal vertebral body, which has dense longitudinal and transverse bone trabecula patterns. FIG. 10B shows an X-ray image of the vertebral body having osteoporosis in its initial stage, in which the image density of the bone is lower as a whole than the image density of the X-ray image shown in FIG. 10A, and in which the patterns of the trabeculae of the bone are thinner and smaller than those shown in FIG. 10A. FIGS. 10C, 10D, and 10E respectively show X-ray images of the vertebral bodies having the osteoporosis in its stages of degree I, degree II, and degree III. As the degree of the osteoporosis becomes higher, the densities of the bone trabeculae in the longitudinal and transverse directions become lower. In this manner, the abnormal states in the images of the trabeculae of the bones are rated with four ranks, i.e. the initial stage, degree I, degree II, and degree III. However, with this method for making a diagnosis of osteoporosis, which relies upon the physician's subjective point of view, the state of the trabeculae of the bone cannot be ascertained quantitatively.
In view of the above circumstances, a method has been proposed, wherein an image signal representing a radiation image of a bone is subjected to spectral analysis utilizing the fast Fourier transform (FFT) and the maximum entropy method (MEM), and the patterns of trabeculae of the bone are numerically expressed with a peak spatial frequency and a power spectrum value. Such a method is described in, for example, "Spectral Analysis of Bone Trabecula Image--Fundamental Experiments and Simulation," by Takigawa, et al , the Magazine of the Japanese Society of Radiological Technology, 1990, 9.10, pp. 1659-1669. Also, a method has been proposed, wherein a power spectrum of a bone image is obtained, and information about the density of trabeculae of the bone is analyzed in accordance with the RMS value (the amount of fluctuation in image density) and the first-order moment (the coarseness and fineness of the image), which are obtained from the power spectrum. Such a method is described in, for example, "Spectral Analysis of Trabecular Pattern," the Magazine Vol. 9, No. 1 (1992), pp. 32-40.
With the aforesaid methods utilizing the spectral analyses, errors in making diagnoses of the osteoporosis can be prevented from occurring due to dependence upon the physician's subjective point of view, and the information about the density of a bone, particularly the bone trabeculae, can be obtained quantitatively.
However, with the aforesaid methods utilizing the spectral analyses, a region of a predetermined range is set on the radiation image, and the mean-level spectrum in the predetermined range is obtained. Therefore, of the information representing the radiation image, only the information corresponding to the predetermined range can be obtained. Accordingly, with the aforesaid methods utilizing the spectral analyses, local spectral analyses of bone images cannot be carried out. Also, with the aforesaid methods utilizing the spectral analyses, wherein the spectral analyses are carried out, the region is set manually. Therefore, the problems often occur in that the region is set at an incorrect location. In such cases, accurate information about the density of the trabeculae of the bone cannot be obtained.